Conventionally, a semiconductor optical device comprising an electro-absorption type light modulator and a semiconductor laser that are integrated on a substrate has been used as a light source for digital optical communication of 2.5 Gb/s to 40 Gb/s. FIG. 28 is a schematic perspective view illustrating a principal part of a conventional semiconductor optical device. FIG. 29 is an enlarged schematic perspective view illustrating a structure of a light modulator region of the semiconductor optical device.
Referring to FIG. 28, a semiconductor optical device 1 has a laser region 2, a light modulator region 3, and an isolation region 4 between the laser region 2 and the light modulator region 3. An electrode 15 is placed at the light modulator region 3, and an electrode 16 is placed at the laser region 2.
The laser region 2 has a diffraction grating and constitutes a so-called distributed feedback laser. In this distributed feedback laser, light wavelength easily varies due to light reflected at a light emitting facet, i.e., return light. For this reason, a window region 5 having no optical waveguide is usually provided continuously with the light modulator region 3.
More specifically, as shown in FIG. 29, in the light modulator region 3, a buffer layer 7 serving as a lower cladding layer, a first light confinement layer 8, an active layer 9, a second light confinement layer 10, a first upper cladding layer 11, and a second upper cladding layer 12 are successively disposed on a substrate 6. In the window region 5, a buried layer 17, a hole trap layer 18, and the second upper cladding layer 12 are successively disposed on the substrate 6. A contact layer 13 and an insulating film 14 are disposed on the second upper cladding layer 12. In other words, the window region 5 has no active layer 9 and no optical waveguide.
By providing the window region 5, a spot diameter of light having passed through the light modulator region 3 spreads. That is, the light advances radially. Therefore, the rate of light that is reflected at a light emitting facet 19, i.e., a facet of the window region 5, and returns to the optical waveguides in the light modulator region 3 and the laser region 2 is reduced. As a result, variations in light wavelength are suppressed.
By the way, the light having passed through the light modulator region 3 goes into the window region 5. At this time, the following problem arises from the radial spread of the light.
There is a case in which the light radially spreading leaks out of the window region 5, that is, the light radially spreading goes from the window region 5 into a region 20 comprising, for example, a buried polyimide layer, which is adjacent to the side portion of the window region 5. In such a case, since the window region 5 and the region 20 usually comprise different materials, the light is reflected at the interface of the window region 5 and the region 20, which causes irregular reflection of the light in the window region 5, leading to deterioration of a beam shape of light that is emitted from the emitting facet 19. This means that the loss of the light at the window region 5 increases. Incidentally, in a conventional example, the loss of the light became about 50%. In the case where the loss of the light is considerable as described above, connection to an optical system cannot be sufficiently performed when the semiconductor optical device 1 is used in optical communication.